The present invention relates to a cylinder head gasket for an internal-combustion engine of the type including a disc of a fibrous and/or porous soft material which may be metallically reinforced and which is provided with a metallic casing, preferably around the combustion chamber passages, the casing having arms which are angled on one or both sides of the disc in the direction toward the soft material. In addition, gaskets of the type to which the invention is directed are impregnated with a saturation agent which is plastic or elastic in its final state and which fills either completely or in part the pores of the soft material except for the regions below the casing.
In practice, soft material cylinder head gaskets for internal-combustion engines are usually made of asbestos fiber webs which are possibly reinforced by embedded rough sheet metal plates. Particularly to increase their hardness and sealing properties, such cylinder head gaskets are impregnated with agents which in the final state are plastic or elastic, as disclosed, for example, in German Auslegeschrift [Published Patent Application] No. 23 04 558.
Cylinder head gaskets designed for engines which undergo high stresses, however, should use soft material having a high yield point in regions which undergo the greatest stress from the compressive sealing forces. The compressive sealing forces are greatest under the flanges which are preferably applied around the combustion chamber passages, since here the thickness of the cylinder head gasket is increased by twice the thickness of the flange metal. Since, however, impregnating agents in the soft material reduce the yield point of the latter, the regions below the casings are kept free of impregnating agent, as disclosed in U.S. Pat. No. 3,970,322.
In the manufacture of such cylinder head gaskets, the completely cut and flanged blank sealing discs are impregnated, preferably by rolling, immersion or spraying, and the impregnating agent is then polymerized to its plastic or elastic consistency, preferably by heating. In this relatively simple process, the regions underneath the casings are covered by the flanges so that no impregnating agent can penetrate thereinto. Nevertheless, it may happen here and there during mass production that in some cylinder head gaskets impregnating agent will reach the area underneath the casings. This results in costly cylinder head gasket rejects which, due to the fact that the yield point of the soft material underneath the flanges is too low, are unsuited for use, particularly in very highly stressed engines.
As disclosed in German Auslegeschrift No. 24 35 957, it has already been proposed to create a sort of block against the penetrating impregnating agent by bending the ends of the flange arms at right angles into the soft material. As that publication discloses, however, this does not completely prevent the flow of impregnating agent in the embodiment illustrated in that publication and in the regions underneath the casings there is merely created a zone which has a low content of impregnating agents.
Tests have shown, however, that even small quantities of impregnating agent reduce the yield point of the soft material, even relatively more than if the soft material had been impregnated throughout. Obviously, such small quantities of impregnating agent are subject to a greater thermal insulation by the surrounding metal casing so that thermal treatment does not completely cross-link the agent located below the casings and that agent acts as a still liquid mass on the surrounding asbestos fibers like a sliding, or lubricating agent. The soft material under the casings will then yield more easily when the gasket is installed in an engine and is subjected to pressure stresses.